Multilayer monolithic ceramic capacitors, often referred to as chip capacitors, usually consist of alternate layers of electrically non-conductive ceramic dielectric material which separate alternately polarizable refractory, electrically conductive metal electrodes. The structure is generated in a "green" state and fired. The ceramic provides not only the dielectric layers but also the mechanical matrix for the electrodes and the encasement system which affords the unit its physical geometry and environmental protection.
Layers of electrode material commonly extend to opposite ends of the capacitor and are interconnected at the ends by a metal coating composition, usually a noble metal such as silver combined with a glass, which is also fired, and thereby bonded to the ends of the capacitor. The metal coating composition on the ends not only connects each electrode layer of like polarity but also provides a solderable media. Solder is commonly used to attach leads to the capacitor or to directly connect the capacitor to a circuit substrate.
The performance of the capacitor is established by the dielectric within the electric field region. Since it is the electric field or active region which accepts the charge, withstands the high potential gradient, and stores the electrical energy, it follows that it is the region in which capacitance failure or degradation is most likely to occur.
These ceramic capacitors are, for the most part, readily manufactured within acceptable capacitance tolerances although it is not atypical for a particular batch to have capacitance values which vary over a wide range of values. In applications where close accuracy in capacitance values is required, e.g., tolerances of plus or minus 1 percent from the rated value, or where in situ adjustment of capacitance value is desired, typical procedure is to individually test and select capacitors having a capacitance value greater than the desired value and to trim these capacitors down to the desired value by removing portions of the electric field region either by cutting and/or by sand-blasting into the structure. Several systems for selectively removing electrode material while monitoring the capacitance value of the capacitor are known in the prior art. Examples of these are U.S. Pat. Nos. 2,603,737 and 2,712,172. Other examples of the state of the art are U.S. Pat. No. 3,235,939 which shows multilayer capacitors calibrated by grinding away a portion of the electric field region from a side edge of the capacitor; U.S. Pat. No. 3,456,170 which shows electrode material being dished out from one of the plane surfaces of the capacitor and an insulating glaze placed over the exposed electric field region; and, British Patent 1,180,928 which teaches the capacitor being made so as to have a plurality of electrode areas of discrete size which are successively cut out as the capacitance is monitored. A still more recent development includes U.S. Pat. No. 3,648,132 which discloses layers of electrode material embedded entirely within the capacitor so that their margin portions are positioned short of the edge. By removing some of the dielectric material which separates the concealed electrode margins from the edge, the concealed electrodes can be exposed and then electrically connected to the electrodes terminating at that end.
In each of the aforementioned prior art systems either the electric field region or the immediately adjacent dielectric region is abraded or otherwise disturbed. In some of the prior art processes, the outer dimensions of the capacitor are changed during adjustment by varying amounts which results in a nonuniform product. More importantly though, by exposing the nascent electric field region to foreign environmental elements, the dielectric strength and other essential properties of the capacitor are seriously impaired. To offset these objections somewhat, U.S. Pat. Nos. 3,456,170 and 3,394,386 have suggested placing a ceramic material over the exposed region and firing it. However, the possibility for damage arises when the critical electric field region is first exposed. In addition to possible deleterious effects, each of these prior art systems involve considerable processing labor and equipment which makes it quite expensive to perform an adjustment in capacitance value. Since certain of the adjustment devices require that electrical contact be made so as to monitor capacitance value during the grinding process it is evident that the instrumentation must be quite sturdy to withstand the constant vibration and abrasive atmosphere.
A variation in the art of incrementally adjusting monolithic capacitors which permits adjustment without the above problems and without invading the structure of the capacitor is U.S. Pat. No. 3,586,933. This patent teaches adjustment by means of serially connecting or disconnecting sets of fine trimming electrodes which extend to a side surface of the capacitor. The drawback of this otherwise noteworthy invention is that connection of the fine trimming electrodes is accomplished by dipping the entire capacitor into a bath of liquid conductive coating material. Thus, while the capacitor can be adjusted by the manufacturer to bring it within design tolerances, it can not be adjusted in situ by the user.
An advance in the art which overcomes both the aforementioned problems and permits in situ adjustment was made with U.S. Pat. No. 3,898,541 which teaches a capacitor having a set of incremental adjusting electrodes extending to a side surface and a connecting electrode which extends to both a side surface and an end terminal. Adjustments according to this invention are made by electrically interconnecting one or more of the adjusting electrodes with the connecting electrode. However, capacitors made in accordance with the teachings of this invention have the practical disadvantage of limiting the minimum capacitance available relative to the base capacitance value before any adjustments are made. This is due to the capacitance effect between the connections, electrode(s) buried inside the capacitor body and any opposing polarity electrodes present inside and outside the capacitor body. Those skilled in the art will readily appreciate the magnitude of this disadvantage since in many applications it is unacceptable for the adjustment electrodes to add to the rated value of the capacitor.
Other patents on the general subject of adjustable capacitors are U.S. Pat. Nos. 2,919,483, 3,223,905, 3,237,006, 3,448,355, 3,883,937, 3,821,617, 3,818,287, 3,400,312, 3,444,436, 2,395,442, 3,496,434, 3,398,541, 3,539,949, 3,714,530, 3,593,115, 3,737,805, 2,736,080, 3,379,943, 3,398,326, 3,448,355, 2,875,387, 3,651,548, 3,896,354, 3,586,933, 3,237,006, 3,883,937 and 3,391,312.